The present invention relates to a vacuum drying apparatus and a vacuum drying method, and more particularly, to a vacuum drying apparatus and a vacuum drying method in which a time required for drying can be reduced, and a drying surface of an object to be dried is satisfactory.
In the case of a color filter for an LCD, for example, a glass substrate is applied with coating liquid such as resist liquid and dried to form a desired pattern by such a photolithography or the like. As coating liquid application system, for example, a spin coating system, a knife coating system, a roll coating system, a bead coating system, or the like can be employed. In any case of application such as mentioned above, a drying process for drying a coating film is required to be taken before a pattern forming process. Conventionally, for an object to be dried such as a glass substrate coated with coating liquid, heat drying has been performed in an oven or a hot plate or the like.
The above method by heating requires long time for drying, and accordingly, in the manufacturing process of a color filter for an LCD as described above, the drying step of the glass substrate coating film has been the rate-determining step for the whole process. Accordingly, in recent years, a vacuum drying apparatus has come to be used so as to make it possible to reduce the time for the drying step. This is a system in which a glass substrate having a coating film is placed in a vacuum state to elevate the solvent evaporation rate to a remarkable extent.
However, even by using such a vacuum drying apparatus, it is invariable that the drying step occupies the rate-determining step for the whole process, and therefore, a further reducing of the time for the drying step is an important subject On the other hand, in the manufacturing process of the color filter for an LCD, the requirement is not only the reducing of the drying time but also the smoothness of the surface of the dried coating film on the glass substrate. In case of the drying by a simple rapid pressure reduction, irregularities are formed on the surface of the coating film to make the product not practically usable.
In view of the above circumstances, the present invention has been made, and its object is to provide a vacuum drying apparatus and a vacuum drying method wherein it is possible to reduce a drying time for an object to be dried and the surface condition of the object after drying is extremely satisfactory.
In order to attain the above object, a first invention of the vacuum drying apparatus comprises: a vacuum chamber provided with an exhaust port; a vacuum pump connected to the exhaust port of the vacuum chamber through a suction pipe; an alternating current motor for driving the vacuum pump; and a frequency converter provided on the input side of the alternating current motor.
Furthermore, the above vacuum drying apparatus further comprises a controller for detecting the vacuum degree in the vacuum chamber, adjusting the frequency converter by the preset vacuum degree to change an alternating current frequency to be introduced into the alternating current motor.
A second invention of the vacuum drying apparatus comprises: a vacuum chamber provided with an exhaust port; a vacuum pump connected to the exhaust port of the vacuum chamber through a suction pipe equipped with a shut-off valve; and a motor for driving the vacuum pump.
Furthermore, the above vacuum drying apparatus further comprises a controller for detecting the vacuum degree in the vacuum chamber, adjusting the shut-off valve by the preset vacuum degree to change an exhaust rate from the exhaust port.
A third invention of the vacuum drying apparatus comprises: a vacuum chamber provided with an exhaust port; a vacuum pump connected to the exhaust port of the vacuum chamber through a suction pipe equipped with a shut-off valve; an alternating current motor for driving the vacuum pump; and a frequency converter provided on the input side of the alternating current motor.
Furthermore, the above vacuum drying apparatus further comprises a controller for detecting the vacuum degree in the vacuum chamber, adjusting the frequency converter by the preset vacuum degree to change an alternating current frequency to be introduced into the alternating current motor, and/or adjusting the shut-off valve by the preset vacuum degree to change an exhaust rate from the exhaust port.
The vacuum drying method of the present invention is a vacuum drying method for placing a substrate coated with coating liquid containing solvent in a vacuum chamber, and evaporating the solvent in the coating liquid under reduced pressure, comprising:
an air eliminating step of reducing the pressure in the vacuum chamber to an exhaust rate changing pressure which is a pressure level slightly higher than the pressure at which the evaporation rate of the solvent in the coating liquid is rapidly elevated,
a solvent evaporating step of reducing the pressure from the above exhaust rate changing pressure to the terminal pressure which is a pressure at which the evaporation of the solvent is completed, and
an atmospheric pressure step of recovering the atmospheric pressure from the above terminal pressure,
wherein the exhaust rate in the air eliminating step is set to a rate faster than the exhaust rate in the solvent evaporating step.
In the vacuum drying method of the present invention, the exhaust rate in the above air eliminating step is set to a rate faster than the exhaust rate in the above solvent evaporating step. Therefore, by making the exhaust rate extremely fast in the air eliminating step which does not affect the evaporation of the solvent contained in the coating liquid and making the exhaust rate in the solvent evaporating step which gives large effect on the smoothing of the coating surface into a speed necessary for smoothing, it becomes possible to expect, as a whole, to make the vacuum drying rate high and the coating surface smooth.
According to the present invention, the exhaust rate in the vacuum chamber at the time of the drying is made selectable in two steps, i.e., at first, the gas in the vacuum chamber is exhausted at a high rate to the vacuum degree which is slightly lower than the vacuum degree at which the evaporation rate of the solvent of the coating film is abruptly elevated, and next, the gas in the vacuum chamber is exhausted at a low rate to cause the solvent of the coating liquid to evaporate gradually. Accordingly, in the first stage exhaust above, it becomes. possible to reduce the drying time, and in the second stage exhaust, it is possible to make the coating surface quality uniform. In addition, in the vacuum drying apparatus of the present invention, by adjusting the frequency converter to change the alternate current frequency to be inputted to the alternate current motor for driving the vacuum pump, and/or by adjusting the shut-off valve provided on the suction pipe to change the exhaust rate from the exhaust port, the exhaust rate of the gas in the vacuum chamber can be optionally controlled. Therefore, it is possible to set in advance a vacuum degree that becomes a boundary between the first stage and the second stage in the above exhaust rate and to change over the exhaust rate from high rate to low rate, thereby reducing the drying time and making the surface condition after drying of the object to be dried extremely satisfactory.